1. Field of the Invention
The present invention relates to an aluminum pigment and process for producing the same. In particular, the present invention relates to an aluminum pigment used in high-quality metallic paints including, for example, high-quality metallic paints for automobile bodies and automotive parts, metallic paints for automobile repairing, metallic paints for household electric appliances, high-quality metallic paints for industrial uses and the like, high-quality metallic printing inks for gravure printing, offset printing, screen printing and the like for plastics and for other uses.
More particularly, the present invention relates to an improved aluminum pigment capable of giving a higher than conventional reflectivity and a very high substrate-hiding power to paint films, prints, plastic films and other materials described above by ordinary application methods.
2. Discussion of the Related Art
Aluminum pigments are generally characterized by their unique metallic impression not possessed by other pigments and their excellent substrate-hiding power and have been widely used for the above-mentioned purposes.
In recent years, automobile appearance has become important. In fact, it has become fashionable to consider the value of the body coating of the automobile equivalent to or even higher than that of the basic functions thereof. One body coating of the past several years appears to possess a white color and a glaring silver metallic tone which has become unpopular, while a soft metallic tone with a pearly luster has gained striking popularity.
There are various types of paints which give a soft metallic tone. Fundamentally, all of these paints contain a special pigment utilizing a light-interfering action (this special pigment is hereinafter referred to as a pearl pigment). A pearl pigment, however, has little or no substrate-hiding power because it is generally transparent and transmits light. Hence, an undercoating step has been necessary prior to the application of a paint containing a pearl pigment in order to hide the substrate. Recently, attempts have been made to eliminate the undercoating step by making a coating by mixing the pearl pigment with an aluminum pigment. This coating, however, has a serious problem in that the mixing impairs the soft metallic tone characterizing the pearl pigment. The problem is caused by the aluminum pigment which is mixed with the pearl pigment.
Two technical factors are necessary in order to impart satisfactory hiding power without impairing the soft metallic tone characterizing the pearl pigment. One technical factor is to minimize the amount of the aluminum pigment to be mixed with the pearl pigment. To achieve this, an aluminum pigment capable of imparting a very high hiding power when used in a small amount is required. The other technical factor is that the aluminum pigment to be mixed with the pearl pigment must have a high reflectivity.
It is generally thought that the soft metallic tone characterizing the pearl pigment is given by the light-interfering action possessed by the pearl pigment. However, the pearl pigment itself has a very low or substantially no light-reflecting ability because the pigment transmits most light. The light-reflecting ability is possessed by the aluminum pigment mixed into the pearl pigment. When the light reflectivity of the aluminum pigment is low, the soft metallic tone of the pearl pigment is not obtained and only a black opaque color tone or appearance having a low commercial value is obtained.
Thus, in order to impart a desired hiding power without impairing the soft metallic tone characterizing the pearl pigment, an aluminum pigment having a high hiding power and a high light reflectivity is required.
In general, an aluminum pigment of higher hiding power has a lower reflectivity. Therefore, it has been difficult to provide an aluminum pigment excellent in both hiding power and reflectivity.
As stated above, as long as any aluminum pigment according to the prior art is used, the soft metallic tone inherently possessed by the pearl pigment is lost. Therefore, it is strongly desired to develop an aluminum pigment suitable for use with the pearl pigment.
A few attempts have been made as follows to solve the above problem. For example, a method wherein an aluminum powder is subjected to surface grinding by wet ball milling to obtain an aluminum pigment of relatively small particle diameter having a high reflectivity is taught by Japanese Patent Application Kokai No. 14358/1974 and an aluminum pigment having a high reflectivity is obtained using an apparatus described in U.S. Pat. No. 3,995,815 (Japanese Patent Publication No. 500504/1980). An aluminum pigment of small particle diameter having a high hiding power is obtained by effecting grinding for a long period of time using a similar apparatus (Japanese Patent Publication No. 17142/1984).
However, none of the aluminum pigments obtained by the above methods possesses a hiding power and reflectivity suitable for use with a pearl pigment.